Gas turbine engine discharge ducts and nozzles can carry gas in the order of 2500 C. during operation. It is conventional to supply liners to protect the underlying structure from the high exhaust gas temperature. It is furthermore conventional to cool these liners with a flow of cool air from a selected location in the compressor or engine by-pass fan duct.
Film cooling which establishes the layer of relatively cool air along the wall is known to be effective. Various approaches have been used with holes through a thin liner which introduced the air. Even openings located with flow perpendicular to the surface are effective. Other approaches introduce the air at an angle in the direction of the gas flow and these methods have shown more effective cooling. It is, however, difficult to establish that direction of cooling air with the very thin liner which is used.
High cooling effectiveness is also found in designs using louvers which extend along the surface transverse to the gas flow. These louvers overlap the liner section with the cool air being introduced between the overlapping member and the base member. Such air introduction is therefore parallel to the downstream surface with no significant radial flow component. Such introduction of the cooling air has been very effective.
The louver construction, however, requires a large number of pieces to establish the overlapping section. It also steps the diameter at each louver or must otherwise be based on a conical section to compensate for these steps. The dimensional control of the outlet has been difficult because of distortion of the overlapping portion at the louver outlet. This results in erratic variation of the slot height and mass flow ratio out of the slot. Consequently, the film effectiveness level and surface temperature are subject to undesirable variations.